It is known that at low speeds, i.e., speeds used for life tests, that the surfaces of ball bearings are in a boundary lubrication mode as opposed to a hydrodynamic lubrication mode. The differences between hydrodynamic and boundary lubrication are well established and are briefly summarized as follows. Under hydrodynamic lubrication, the bearing friction is due to the viscosity of the lubricant which completely separates the relatively moving metal surfaces and this occurs at high speeds with low loads. At lower speeds and high loads the metal is separated by a layer of only one or two molecules thickness which is generally less than the dimensions of point asperities, and some degree of metal to metal contact may occur. For this reason, long chained organic molecules are effective boundary lubricants. Greatest protection occurs if the organic lubricant is either a solid adsorbed on the surface or a compound which chemically bonds to the metal surface.
Also of interest is the influence of chain length on the effectiveness of a boundary lubricant. It was shown that the coefficient of friction decreases from 0.18 to 0.05 as the chain length is increased from 6 to 14 carbon atoms. Although the coefficient of friction is a minimum at a chain length of 14 carbon atoms, it does not increase significantly as the chain length is increased further.
It is also noted that the mechanism of lubrication for the boundary condition results in lubricant deformation and shear. That is, the high forces created at the asperities (be they due to shear, temperature, pressure, sudden or shock loading or some other form of energy) result in shear of the lubricating film. This shearing or scission at the molecular level produces, for example in the case of fatty acids, additional organic acids which are themselves beneficial.
In addition to the above, it was determined that it would be beneficial to have a boundary lubricant additive soluble in the fluorinated base oil. Fluorinated fluids, such as perfluoropolyethers are generally considered hydrodynamic, not boundary lubricants. It was determined, therefore, to examine those molecular structures which could function as a boundary lubricant and at the same time be soluble in the fluorinated base oil.
It was found that a structure having a fluorocarbon chain particularly those containing 8 or more carbon atoms for solubilization, and having a functionalized head which was a hydrogen containing moiety having some level of acidity was especially useful.
Reference may be had to U.S. Pat. No. 3,367,868 dated Feb. 6, 1968 for a disclosure of perfluoropolyether lubricants useful in the present invention. The lubricants particularly useful herein have a maximum volatility of 50% at 400.degree. F. according to Federal Test Method Standard -791, method 351 and a maximum pour point of 50.degree. F. These values do not apply to all useful fluorinated lubricating fluids, but apply to the preferred component (a). Other references of interest and relating to these lubricants include U.S. Pat. No. 3,242,218 to Miller; U.S. Pat. Nos. 3,306,853; 3,306,854; 3,306,855 all dated Feb. 28, 1967; U.S. Pat. No. 3,445,392 dated May 20, 1969; U.S. Pat. No. 3,505,229 dated Apr. 7, 1970; U.S. Pat. No. 3,901,700 dated Aug. 26, 1975; U.S. Pat. No. 4,529,659 dated Jul. 16, 1985. The disclosures of these patents insofar as they relate to perfluoro polyether lubricants are incorporated herein by reference.
For reference to various poly- and per-fluorinated compounds useful in carrying out the present invention as component (b) reference may be had to U.S. Pat. No. 3,965,148 dated Jun. 22, 1976. Polyfluorinated alcohols as additives in certain synthetic oils (e.g., di-2-ethylhexyl sebacate) are disclosed as antifriction compounds in an article by Sekiyu et al, Department of Chem. Eng. Tokyo Institute of Technology, Tokyo, Japan (Sekiyu Gakkaishi 1986, 29(2) 183-6). Polyfluorinated alkanols particularly useful in carrying out the present invention are also disclosed in U.S. Pat. No. 3,283,012 dated Nov. 1, 1966.
A principal purpose of this invention is to reduce wear in bearings which are run in an environment of highly fluorinated fluids. These fluids include, but are not limited to perfluoralkyl ethers (marketed under trademarks including AFLUNOX.TM., DEMNUM.TM. (Daikin) KRYTOX.TM. (DuPont) and FOMBLIN.TM. (Montedison), perfluoroalkanes, and perfluoroamines (such as 3M's FLUORINERT.TM. Series and Imperial Smelting Corp's PP Series) and other perfluoro and highly fluorinated compounds.
Utility of compositions containing components (a) and (b) has been demonstrated for ball bearings, but has potential application for other types of bearings and any case where two relatively moving surfaces may otherwise make contact in a highly fluorinated fluid.
Prior art methods employ either no additive lubricant or an additive lubricant insoluble in highly fluorinated fluids. The additives described in this invention are soluble in highly fluorinated fluids.
One prior art method used the highly fluorinated fluid without an additive. However, this resulted in unacceptable wear levels in some systems and variable wear levels in other systems. The other prior art method pretreated the surfaces with a hydrocarbon type lubricant which was not fluorinated. However, hydrocarbon lubricants are virtually insoluble in the highly fluorinated fluids.
The disadvantages of the former methods are that where a two-phase, non-homogeneous system is used, wear occurs. Use of an insoluble lubricant provides only a limited amount of lubricant because that which is applied to the surface during fabrication is the only material available during operation. Through the use of a soluble additive during operation, lubricant is continuously available to the bearing. It is also possible for an insoluble lubricant to be displaced by the highly fluorinated fluid during operation. The step of carefully applying a known amount of lubricant during fabrication can be avoided. If no additive is added then excessive wear occurs.